Boron nitride coated boron filaments

ABSTRACT

A method for surface nitriding boron filaments to make the filaments useful as reinforcement agents in composite materials. The method involves initially forming a liquid boron oxide coating on the filament, for example, by heating the filament at temperatures of from about 560* C. to 800* C. in an oxidizing atmosphere and then converting the liquid oxide coating to a solid, continuous boron nitride coating by, for example, heating the filament at temperatures of from about 600*C. to 1,100* C. in an nitrogen-containing atmosphere.

United States Patent Inventors Jose L. Camahort Millbrae; Mario P.Gomez, Sunnyvale, both of Calif. Appl. No. 38,648 Filed May 22, 1970Patented Jan. 11,1972 Assignee Lockheed Aircraft Corporation Burbank,Calif.

Original application Aug. 19, 1968, Ser. No. 753,589, now Patent No.3,573,969, dated Apr. 6, 1971. Divided and this application May 22,1970, Ser. No. 38,648

BORON NITRIDE COATED BORON FILAMENTS 3 Claims, 1 Drawing Fig.

U.S.Cl 117/169 R,

117/106 R, l17/D1G. 10 Int. Cl B4441 1/42 Field of Search 1 17/106 R,

169,128,231,DIG.10,100B,l00M,118,69

LlLTlMATE TENSILE STRENGTH (pSi X10 [56] References Cited UNITED STATESPATENTS 2,865,715 12/1958 Kamlet 117/D1G. 10 3,321,337 5/1967 Patterson117/106 X 3,410,715 12/1968 Hough 117/128 3,451,840 6/1969 Hough 117/106X Primary Examiner-Alfred L. Leavitt Assistant Examiner-Wm. E. BallAttorneys-Richard H. Bryer and George C. Sullivan ABSTRACT: A method forsurface nitriding boron filaments to make the filaments useful asreinforcement agents in composite materials. The method involvesinitially forming a liquid boron oxide coating on the filament, forexample, by heating the filament at temperatures of from about 560 C. to800 C. in an oxidizing atmosphere and then converting the liquid oxidecoating to a solid, continuous boron nitride coating by, for example,heating the filament at temperatures of from about 600C. to 1,100 C. inan nitrogen-containing atmosphere.

CURVE 2 TIME IN MINUTES BORON NITRIDE COATED BORON FILAMENTSCROSS-REFERENCE TO RELATED APPLICATIONS This is a division ofapplication Ser. No. 753,589, filed on" Aug. I9, l968,now US. Pat. No.3,573,969.

BACKGROUND OF THE INVENTION Interest in composite materials whereinvarious types of high-strength, high-modulus filaments, both metallicand nonmetallic, are incorporate in metal matrices to enhance thestrength and stiffness properties of the metal matrices has greatlyincreased in recent years. Commercially available filaments of boronpossess properties which make them quite attractive as reinforcementagents for structural composites. For example, a typical 4-mil diameterboron filament, made by heating a 0.5-mil diameter tungsten wire in anatmosphere of boron chloride-hydrogen exhibits an average strength of450,000 p.s.i., an elastic modulus of 60 million p.s.i. and a density of0.095 pounds per cubic inch. Resin matrix boron composites are currentlyused as structural parts in aerospace applications. Resin matrices,however, have low mechanical strength and also fail in the area ofhigh-temperature applications since they decompose at moderatetemperatures.

Consequently, it would be advantageous to use metal and metal alloymatrices that not only possess good mechanical strength but also permitthe use of such composites at much higher temperatures then resinmatrices due to their higher melting point. Heretofore, it has beenimpossible, however, to directly incorporate boron filaments into metaland metal alloy matrices by liquid metal processing techniques. Thechemical reactivity of boron with structural metal matrices such asaluminum, magnesium, nickel, titanium, iron, berylliurn, chromium andalloys thereof is well documented. For example, the prolonged contact ofaluminum and boron at temperatures in the order of 400 to 500 C. resultsin the degradation of the boron filaments due to formation of thebrittle compound aluminum boride. This compound, and other brittle metalborides, cause premature fracture of the boron filaments upon theapplication of stresses. The usefulness of boron filaments in metalmatrices is accordingly minimized since no substantial strengthimprovements are realized.

To minimize such boron filament degradation, metal matrix composites, asnow fabricated by the art, must be formed at low temperatures by usingcumbersome and expensive fusion bonding techniques. By these techniques,the boron filaments are arranged with, for example, aluminum foil insandwich fashion so that boron filaments alternate with layers ofaluminum. A firm bond between the aluminum and boron filaments isachieved by heating and compressing the sandwich. Since the temperaturefor this process of fusion has to be held lower than 600 C. in order toavoid chemical reaction between boron and aluminum, pressuresapproaching l0,000 p.s.i. are required. This is rather impractical forsandwiches of say 1 square foot in size since forces as much as1,500,000 pounds would be needed for compression. Further, the resultingcomposites are severely limited in their high temperature capabilitiesdue to reaction between the boron and metal at temperatures in the orderof 500 C. and higher and the resulting formation of brittle metalborides.

The desirability of using liquid metal processing techniques and hightemperature forming and joining operations to produce structuralcomposite bodies containing boron has long been recognized. To date,however, satisfactory composite bodies formed by this technique have notbeen realized due to boride formation. It has been recognized by the artthat theoretically this difficulty could be overcome by the use of adiffusion barrier between the boron filaments and the metal matricessince such barriers should act to limit or prevent deletrious reactionswhile still providing adequate bonding for effective filament-matrixload transfer. Several metal and inorganic compound coatings have beentested in conjunction with boron filaments, but all such coatings sufferfrom serious disadvantages. A silicon compound coating does not wetmolten metals such as aluminum and fails to provide adequate bondingbetween the boron filament and the metal matrix. Silver and nickel havebeen tested and been found to be ineffectual.

Theoretically, boron nitride should act as a highly satisfactorydiffusion barrier since it is essentially inert to both boron and themetal matrices of interest at elevated temperatures. Heretofore,however, it has been impossible to obtain a satisfactory boron nitridecoating on boron that would prevent degradation of the boron filamentduring fabrication of the composite at temperatures in the order of 500C. and higher and use of the composite at such elevated temperatures.

SUMMARY OF THE INVENTION Briefly, in accordance with the invention,there is described a process for the surface nitriding of boronfilaments, which filaments are then uniquely suitable for incorporationinto various metal matrices. The process of the invention has been foundto result in boron nitride coatings which obviate the aforementioneddifficulties and problems associated with the use of boron filaments asreinforcement materials in metal matrices.

In particular, by the process of the invention, degradation of the boronfilament is essentially precluded during formation of the boron nitridecoating on the boron filament and further during the incorporation ofthe coated filament into metal matrices. Further, the boron nitridecoating substantially acts to minimize diffusion between boron filamentsand metal matrices at elevated temperatures. As a result, metal matricesincorporating boron filaments processed in accordance with the inventionexhibit outstanding properties. For example, boron-aluminum compositerods (0.020 inches in diameter) were prepared by identical liquidinfiltration techniques using as-received boron filaments and boronfilaments nitrided by the process of the invention. The composite rodscontaining the nitrided filaments averaged about 180,000 p.s.i. inultimate tensile strength while those rods containing the uncoatedfilaments averaged only about [10,000 p.s.i. in ultimate tensilestrength.

More particularly, the method of the invention involves initiallyforming a liquid boron oxide coating on the boron filament andsubsequently converting the liquid oxide coating to a solid, continuousboron nitride coating. It has been determined that the efficacy of theprocess is dependent upon this specified sequence of processing steps.Formation of the liquid boron oxide coating is a prerequisite to theobtaining of a continuous boron nitride coating. Applicants have foundit impossible to directly produce a boron nitride coating, for example,by reacting the boron filament with nitrogen or ammonia or to replacethe liquid oxide intermediate coating with a solid boron oxide coatingwithout degrading the filament or forming a nitride film that isineffectual in protecting the filament at elevated temperatures.

A further advantage accruing to the boron nitride coated filaments ofthe invention is that the coating is thick enough to be an effectivediffusion barrier and yet thin enough to have a negligible effect on theboron filament strength. Since boron nitride is much weaker than boron,it is desirable to make the boron nitride coating as thin as possible soas not to substantially decrease the volume fraction of boron in thecomposite material. For example, a one-sixth mil thick boron nitridecoating on a 4-mil diameter boron filament would constitute l4.5 volumepercent of the coated filament with a corresponding loss in filamentstrength given by the law of mixtures:

fn- 'mv+( fmv) where ois the ultimate tensile strength and f the volumefraction.

BRIEF DESCRIPTION OF THE DRAWING The invention may be more easilyunderstood by reference to the drawing which on coordinates of ultimatetensile strength and time in minutes is a semilog plot showingmechanical strength degradation of boron filaments as a function of timeof exposure of the filaments in molten aluminum maintained at 700 C.Curve 1 of the drawing shows the mechanical strength degradation ofboron filaments protected by the boron nitride coating of the invention.Curve 2 of the drawing shows the mechanical strength degradation ofasreceived boron filaments which were not coated in accordance with themethod of the invention. As evidenced by these curves, a rapiddeterioration of a mechanical strength was exhibited by the unprotectedfilaments. The coated boron filaments showed significantly lessdegradation of mechanical strength. The data depicted by this drawingwas obtained by immersing both coated and uncoated boron filaments inthe molten aluminum maintained at 700 C. for one-half, l, 3, 5 andminutes and then tensile testing the filaments. All coated boronfilaments were made by the technique hereinafter described inconjunction with example I of the specification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the presentinvention involves the formation of a continuous adherent boron nitridecoating on boron filaments, which filaments are then uniquely suitablefor incorporation into metal and metal alloys matrices for use asstructural composite materials. The process of the invention fornitriding the surface of boron filaments results in a boron nitridecoating which protects the filaments from chemical attack by the metaland metal alloys matrices and further minimizes diffusion between thefilament and the matrix at elevated temperatures. The method is carried.out by initially forming a uniform liquid boron oxide coating on theboron filament and subsequently converting the liquid oxide coating to asolid continuous boron nitride coating.

The liquid boron oxide coating is readily formed by several techniques.One preferred technique involves heating the boron filament surface inan oxidizing atmosphere such as air, oxygen and mixtures of oxygen withnitrogen or other inert gases. Minimum temperatures in the order of 560C. are required to form the liquid oxide coating required by theinvention. During oxidation of the boron filament amorphous, that is,vitreous boron oxide is formed which does not have a definite meltingpoint. The softening range for this oxide is between 560 and 630 C.Below about 560 C., the viscosity of the oxide is sufficiently high asto make the coating, for the purpose of the invention, a solid oxidecoating. It has been determined that a solid oxide coating does not forma continuous layer around the boron filament and accordingly, does notprevent degradation of the boron filament at elevated tem peratures dueto the boron-metal matrix reaction which results in formation of brittlemetal boride. Additionally, it has been found that degradation of theboron filament at temperatures below about 560 C. during formation ofthe boron oxide coating is a serious problem due to the prolongedreaction times required to form the coating. Boron filaments are knownto degrade with increasing reaction times in oxidizing atmospheres dueto formation of the brittle nonuniform boron oxide.

As temperatures are increased above 630 C., the liquid boron oxidecoating becomes less viscous and tends to flow under the influence ofgravity. Up to about 800 C. this flow, however, is negligible. Above 800C., the flow is such as to cause reduction in the thickness of thecoating. For temperatures in the order of 800 to 1,000 C. and higher,evaporation of the boron oxide coating tends to further reduce thecoating thickness. Since, in accordance with the invention, it isnecessary to form an essentially continuous oxide coating, maximumtemperatures in the order of 800 C. are preferred. However, it has beenfound that a continuous coating results when the coating thickness is inthe order of 0.1 microns. Since coating thickness is readilydeterminable by a chemical analysis and optical microscopy, it is withinthe skill of the art to utilize temperatures above 800 C. provided suchtemperatures result in a minimum coating thickness in the order of 0.]microns. Generally, temperatures in excess of l ,000 C. are to beavoided, however, since in an oxidizing atmosphere, degradation of boronfilament due to formation of brittle nonuniform boron oxide is quitepronounced, to the detriment of the strength characteristics of thefilament. Based on the preceding, a preferred temperature range is from560 to 800 C. with an optimum range being between about 560 to 650 C. Ata temperature of 650 C., a reaction time of about 30 seconds has beenfound adequate for forming a continuous liquid boron oxide coating.Since temperatures and time are interdependent, higher temperatures willrequire shorter times and lower temperatures will require longerreaction times.

A liquid boron oxide coating also may be formed on the filament bypulling the filament through molten boron oxide or by passing thefilament over an evaporating boron oxide melt.

The liquid boron oxide coated filament is then heated in a nitrogencontaining atmosphere such as ammonia and an ammonia-nitrogen mixture toconvert the liquid boron oxide coating to a solid, essentiallycontinuous boron nitride coating. While nitridation begins attemperatures in the order of 350 C., temperatures in the order of 800 C.and higher are preferred since the resulting increased reaction rateimproves diffusion of the nitriding gases into the boron oxide layer andcauses formation of a higher purity boron nitride coating. Generally,temperatures in excess of l,l00 C. are to be avoided, however, sincedegradation of the boron filament at such elevated temperatures becomessignificant. For temperatures in the range of about 800 to l,l00 C. theboron filament undergoes a small nonsignificant amount of degradation.Based on the preceding, a preferred temperature range is from about 600to l,l00 C. with an optimum range being between about 800 to l,l00 C.Within these ranges, it has been determined that a reaction time ofabout 30 seconds at l,060 C. and about 2 minutes at 900 C. issatisfactory in converting the oxide coating to a continuous boronnitride coating. Since temperatures and times are interdependent, highertemperatures will require shorter times and lower temperatures willrequire longer reaction times.

Pure nitrogen atmospheres have been found to be unsatisfactory sincethey require temperatures in the order of l,600 C. to convert the oxidecoating to the nitride coating, and, as previously discussed, suchelevated temperatures cause serious degradation of the underlying boronfilaments. Consequently, nitrogen-containing atmospheres which aresufficiently reactive to cause nitridation in accordance with thepreceding discussion without attack of the underlying boron filament orformation of undesirable byproducts are utilized. Such atmospheres areconsidered within the skill of the art.

As has been previously discussed, a continuous boron nitride coatingresults when the minimum coating thickness is in the order of 0.1microns. As has been previously discussed, it is desirable to utilizethin boron nitride coatings on the boron filaments, since as the volumefraction of the boron nitride coating on the boron filament increases,the ultimate tensile strength of the boron nitride coated filamentdecreases. Generally, applicants have found a coating thickness of 0.7microns to be a maximum, practical thickness in view of the precedingvHowever, thickness above 0.7 microns can be utilized if thecorresponding loss in filament strength is not a significant factor inthe use of the filament. The thickness of the boron nitride coatingdepends on the thickness and flow and evaporation characteristics of theliquid boron oxide coating, which varies as discussed herein with thereaction times and temperatures utilized during the oxidation andnitridation steps. Commensurately, with the discussion relating toformation of the boron-oxide-coated filament, it has been determinedthat for nitriding temperatures in the order of l,000 C., someevaporation of the boron oxide coating occurs prior to its conversion toboron nitride.

A continuous nitridation process has been devised whereby one or moreboron filaments were pulled through two reactors containingrespectively, air and ammonia. A small amount, for example, 5 volumepercent of nitrogen or hydrogen gas is preferably added to the ammoniaatmosphere to prevent excessive cracking of the ammonia at elevatedtemperature. Flow rates of ammonia in excess of the stoichiometry amountare preferably maintained in the nitriding reactor. Suitable reactiontimes are easily achieved by varying the filament takeup speed. Thefollowing reactions are assumed to take place:

The resulting nitrided boron filaments are uniquely suitable forincorporation into structural metal and metal alloy matrices by liquidinfiltration techniques or metal casting methods. A preferred compositefabrication process involves pulling a number of the nitrided boronfilaments through the desired molten metal bath to produce a compositewire or rod wherein the filaments are arranged and aligned properly. Thefilaments enter the molten metal bath through separate holes on anentrance disk which ensures sufficient space between the filaments sothat all filament surfaces are wetted. The filaments then exit togetherthrough an orifice of any desired size and shape. The resultingcomposite rods or wires are then used to produce larger compositebodies, such as sheets and tapes by passing the rod or wire through asecond molten bath of the same metal or a lower melting metal or alloy.For example, a 0.020 inch diameter boron-aluminum composite rod wasproduced by pulling l6 nitrided boron filaments through a moltenaluminum bath. Ten of these composite wires were then pulled through asecond molten aluminum bath to produce a 0.20-inch wide by 0.020-inchthick composite tape. The boron nitride coating formed on the boronfilaments by the process of the invention protected the filament fromchemical attack by the molten aluminum during this liquid infiltrationprocess.

Specific examples of procedures used in the preparation of materials ofthe invention are given below: These examples are to be construed asillustrative only and not as limiting in any manner the scope and spiritof the invention as defined by the appended claims.

Example 1 A single boron filament was pulled continuously through areactor containing air at 650 C. and then through a second reactorcontaining ammonia at 900 C. Reaction times of about 2 minutes in eachreaction were maintained by a filament takeup speed of 0.5 feet perminute through the two 12- inch long quartz reactors. An ammonia flowrate of about 2 in aluminum, aluminum alloy, nickel, titanium and glassycarbon.

EXAMPLE 2 Sixteen boron filaments were passed continuously through twosuccessive l-inch diameter 12-inch long stainless steel reactorscontaining air at 650 C. and an ammonia5 percent nitrogen atmosphere at1,060 C. respectively. A filament takeup speed of about 2 feet perminute resulted in reaction times of about 30 seconds in each reaction.Gas flow rates were 2 cubic feet per hour of ammonia and 0.1 cubic feetper hour of nitrogen in the nitriding reactor. Optical microscopy showeda 0.1 to 0.2 micron thick boron nitride coating on the filaments. Thiscoating showed excellent protective action in molten aluminumduringEliguid infiltration experiments.

AMPLE 3 An experiment similar to that described in example 2 wasperformed with the oxidizing reactor containing air at about 540 C. andthe nitriding reactor containing a mixture of ammonia-5 percent nitrogenat about l,000 C. No effective boron nitride coatings were obtainedunder these conditions using filament takeup speeds of from about 0.5feet per minute to about 2 feet per minute. Such filaments were attackedby molten aluminum during subsequent composite fabrications resulting ina average ultimate tensile strength of only 100,000 to l 10,000 psi. Incontrast, utilizing a temperature of 650 C. in the oxidizing reactor,all other conditions being the same, resulted in a composite having anaverage ultimate tensile strength of from about 180,000 to 195,000p.s.i. This example illustrates the criticality of forming a liquidboron oxide coating at a temperature of about 560 C. or higher.

What is claimed is:

l. A boron filament useful as a reinforcement filament in compositemetal and metal alloy matrices and suitable for incorporation into saidmatrices by liquid infiltration and metal casting techniques andcharacterized by having an adherently bonded, essentially continuoussurface coating of boron nitride at least about 0.1 microns thick.

2. A boron filament in accordance with claim 1 having ultimate tensilestrength characteristics in accordance with curve 1 of the drawing.

3. A boron filament in accordance with claim 1 which is essentiallyinert during liquid infiltration and metal casting techniques and inmetal and metal alloy matrices at temperatures of 500 C. and higher.

2. A boron filament in accordance with claim 1 having ultimate tensilestrength characteristics in accordance with curve 1 of the drawing.
 3. Aboron filament in accordance with claim 1 which is essentially inertduring liquid infiltration and metal casting techniques and in metal andmetal alloy matrices at temperatures of 500* C. and higher.